Mechanism and control of crack generation in glass substrates during crystallization of a-Si Films by flash lamp annealing

We investigate the impact of the materials of glass substrates on crack formation during flash lamp annealing (FLA) of 4.5 μm-thick precursor amorphous silicon (a-Si) films for the formation of polycrystalline Si (poly-Si) films. The use of soda lime glass substrates, with the largest thermal expansion coefficient (α) and the lowest glass transition temperature (Tg) in glass materials attempted in this study, results in the serious formation of cracks on and inside the glass substrates. Cracks are also seen on the surface of quartz glass substrates, which have much smaller α and higher Tg, after FLA. Furthermore, flash-lamp-crystallized (FLC) poly-Si films have linearly-connected low-crystallinity regions only when quartz glass substrates are used. These facts indicate that the expansion of Si films induces cracks in quartz glass substrates, while the expansion of the upper part of glass is the cause of the crack formation in glass substrates with large α. The generation of cracks is most significantly suppressed when we use alkali-free glass substrates, with a moderate α and a relatively high Tg, which will contribute to the realization of high-quality poly-Si films and high-performance solar cells.